Stimulus-responsive gels, which change their volumes by swelling or shrinking according to an external stimulus (pH, temperature, ionic concentration, etc.) given thereon, are highly promising as sensor elements and the like. Conventionally, evaluations of the volume changes have been carried out mainly by volumetric measurement using microscopy, weight measurement using a scale or the like. Such methods are difficult to use for actually constructing a new system using such a stimulus-responsive gel.
A system has been reported recently, which uses a stimulus-responsive gel which makes use of a structural color produced from fine particles such as silica particles arrayed inside the stimulus-responsive gel, thereby to overcome such a problem (see Non-Patent Citation 1, Patent Citation 1, etc., for example). In this method, an external stimulus changes a swelling ratio of the stimulus-responsive gel, thereby to change intervals between the silica particles. The changes in the intervals cause changes in wavelength and intensity of the structural color. In this way, the volume change is converted into an optical change. Moreover, similar structural color can be obtained by forming holes arrayed in order by dissolving away such fine particles arrayed in a stimulus-responsive gel. A system using such a method to convert a volume change to an optical change has been also reported (see Patent Citation 2, for example).
Moreover, even though it does not concern conversion of a volume change in a stimulus-responsive gel to optical data, optical devices and sensors have been reported, which changes light transmittance by a volume change in stimulus-responsive gel particles arrayed between two transparent boards, the volume change being caused by light irradiated on the stimulus-responsive gel particles (for example, Patent Citations 3, 4, 5, and 6, etc.). These reports describe use of the stimulus-responsive gel particles in which color materials are dispersed. The arts described in the reports make use of the phenomenon in which when the stimulus-responsive gel particles are swollen, the gap between the two boards is filled with the swollen stimulus-responsive gel particles. As a result, the entire light will be absorbed by the pigment or the like in the gel particles, thereby causing a coloring state. On the other hand, when the gel particles are shrunk, the gel particles occupies very small portion of the gap between the two boards, whereby the most of the light will not be absorbed therein and pass therethrough, thereby causing a colorless state.
[Patent Citation 1]    Japanese Translation of PCT International Application, Tokuhyo, No. 2001-505236 (published on Apr. 17, 2001)
[Patent Citation 2]    Japanese Unexamined Patent Application Publication, Tokukai, No. 2004-27195 (published on Jan. 29, 2004)
[Patent Citation 3]    Japanese Unexamined Patent Application Publication, Tokukai, No. 2000-266676 (published on Sep. 29, 2000)
[Patent Citation 4]    Japanese Unexamined Patent Application Publication, Tokukaihei, No. 11-228850 (published on Aug. 24, 1999)
[Patent Citation 5]    Japanese Unexamined Patent Application Publication, Tokukai, No. 2001-33832 (published on Feb. 9, 2001)
[Patent Citation 6]    Japanese Unexamined Patent Application Publication, Tokukai, No. 2005-10490 (published on Jan. 13, 2005)
[Non-Patent Citation 1]    J. H. Holtz, S. A. Asher, Nature, 389, 829-832 (1997)